This is Mazon Monday post #193. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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The Mazon Creek fossil family has a new member... Squirmarius testai. It was described in the paper "A reappraisal of Nemavermes mackeei from the Mazon Creek fossil site expands Carboniferous cyclostome diversity". The paper's lead author is Victoria McCoy, who you'll remember from her work on Tullymonstrum gregarium. S. testai was named for Tom Testa, long time, prolific Mazon Creek collector. Tom has a few other species named for him - Testajapyx thomasi and Testaneura testai, both insects. Jack Witty, Hamed Sadabadi, and Paul Mayer were co-authors of the paper. Squirmarius testai is a cyclostome, which is a jawless fish. The tazon Cyclostomi also includes hagfish and lampreys.
Abstract
Nemavermes mackeei Schram, 1973, found in the Mazon Creek fossil site and the Bear Gulch Limestone, was described initially as a free-living marine nematode. Here we investigate 13 specimens of N. mackeei from the Mazon Creek to reassess its morphology and identity, and also two specimens originally identified as Gilpichthys greenei Bardack and Richardson, 1977. Based on the extensive morphological variation among these specimens, N. mackeei encompasses multiple species that are only distantly related. The holotype of N. mackeei is a proboscis of Tullimonstrum gregarium Richardson, 1966, making N. mackeei a junior synonym of T. gregarium. However, the other specimens that we investigated could not be attributed to T. gregarium. We name a new species from these specimens: Squirmarius testai new genus new species, a cyclostome. One specimen is likely a juvenile G. greenei. Other specimens were not identified during this study but represent a variety of vermiform bilaterians.
Non-technical Summary
Fossil soft-bodied worms such as nematodes are rare, and, because they often have few features preserved, difficult to interpret. A number of worm-like specimens from the late Carboniferous Mazon Creek fossil site were originally identified as a species of free-living nematode, called Nemavermes mackeei, which was among the oldest and largest fossil free-living nematodes. Here we reinvestigate these specimens, and determine that they encompass multiple species, including worms (but not free-living nematode worms) and cyclostome fish. In particular, some of these specimens belong to a new species of cyclostome fish, which we name Squirmarius testai.
Introduction
Nemavermes mackeei Schram, Reference Schram1973 is an extinct, soft-bodied, vermiform animal that has been found in both the ~308.6–308.4 Ma (Montañez et al., Reference Montañez, McElwain, Poulsen, White, DiMichele, Wilson, Griggs and Hren2016) Mazon Creek fossil site in Illinois, USA (Schram, Reference Schram1973, Reference Schram and Nitecki1979) and the ~330.3–323.4 Ma (Grogan and Lund, Reference Grogan and Lund2002) Bear Gulch Limestone in Montana, USA (Schram, Reference Schram1977, Reference Schram and Nitecki1979). It varies widely in size, ranging from 40–140 mm long and 3–16 mm wide, and was described originally as having a sparse coating of hairs or setae and well-developed labial papillae (Schram, Reference Schram1973). Based on these features, N. mackeei was interpreted to be a free-living marine nematode (Schram, Reference Schram1973), making it one of the oldest and largest fossil free-living nematodes (Poinar, Reference Poinar2011).
Nemavermes mackeei is an element of a nearshore marine chronofauna with characteristic taxa and ecological structure that was relatively stable throughout the Carboniferous (Schram, Reference Schram and Nitecki1979) and might have continued into the Triassic (Briggs and Gall, Reference Briggs and Gall1990). This chronofauna is characterized by malacostracan crustaceans, worms, horseshoe crabs, and pelecypods (Johnson and Richardson, Reference Johnson and Richardson1966; Schram, Reference Schram and Nitecki1979; Baird and Maples, Reference Baird, Maples, Shabica and Hay1997; Grogan and Lund, Reference Grogan and Lund2002), and is distinct from the classic offshore Carboniferous marine fauna, which consists of shelly animals, e.g., corals, attached echinoderms, foraminiferans, ostracods, and stony bryozoans (Baird, Reference Baird, Shabica and Hay1997; Grogan and Lund, Reference Grogan and Lund2002; Clements et al., Reference Clements, Purnell and Gabbott2019). This nearshore marine chronofauna is preserved in Carboniferous fossil sites across North America and Europe, with some variability due to age (there is some faunal turnover) and preservational quality (Schram, Reference Schram and Nitecki1979; Baird et al., Reference Baird, Sroka, Shabica and Beard1985). The brackish-to-marine Essex assemblage from the Mazon Creek fossil site (which also preserves the freshwater/terrestrial Braidwood assemblage) and the Bear Gulch limestone—the two sites from which N. mackeei is known—exhibit some of the best preservation of this chronofauna, and therefore have some of the most diverse fossil assemblages (Schram, Reference Schram and Nitecki1979).
Here we reinvestigate the morphology of specimens identified as Nemavermes mackeei from the Mazon Creek fossil site. Based on the wide morphological variation, these specimens belong to multiple species, none of which is likely to have been a free-living marine nematode due to their large size and lack of nematode features.
The research also found that the holotype of Nemavermes mackeei, which should be reclassified as a junior synonym of Tullymonstrum gregarium. N. mackeei was described by Frederick Schram in 1973. N. mackeei was previously known as a nematode.
Specimens
Figure 1. Specimens of Squirmarius testai n. gen. n. sp.: (1) holotype, FMNH PF 17809; (2) FMNH PF 17810; (3) FMNH PF 17812; (4) FMNH PF 17811; (5) LF 2101; (6) LF 5664, which was collected by David Douglass and donated by the David and Sandra Douglass collection to the Lauer Foundation for Paleontology, Science, and Education in Wheaton Illinois for this study. Scale bars = 10 mm.
